This invention relates to circuitry for detecting ringing and trip currents as well as foreign potentials on telephone lines.
With the advent of electronic switching systems, speedy and efficient program controlled testing and ringing procedures were introduced to check for line and circuit abnormalities on each call and to ensure the proper ringing of called lines. The procedures generally commence with the testing of a called line immediately prior to ringing and for detecting troubles that would impair the ringing job. Such trouble testing typically includes checks for excessive line leakage and for foreign potentials which may be occasioned by crosses between power and telephone lines. After the tests indicate that the line is in satisfactory working order, the procedure continues with periodic 20 Hertz ringing power being connected to the called line and the actual flow of ringing current over the called loop being verified by a ringing current detector. Thereafter, when the rung telephone is answered, the telephone off-hook loop current is detected to interrupt, or trip, further ringing of that phone and to make the ringing facilities available for serving other calls.
One exemplary arrangement for performing the foregoing testing and ringing functions is disclosed in L. F. Goeller, Jr., U.S. Pat. No. 3,378,650 of Apr. 16, 1968. Although the Goeller arrangement has achieved widespread use in present day switching systems, a need yet exists for ringing facilities with reduced power requirements, smaller physical size, integrated self-checking, and more reliable operation in the presence of undesired signals such as induced longitudinal line voltages and harmonic signals.